The use of polymers as a rheology modifier (or viscosity modifier) or as a dispersant in a coating composition and/or an oil of lubricating viscosity is well known. Typically polymers with rheology modifier properties have a number average molecular weight of 25,000 or higher. In contrast polymers with a dispersant property have a number average molecular weight of less than 25,000.
The use of a viscosity modifier or dispersant in an oil of lubricating viscosity to impart desired low and high temperature viscometrics and other viscosity properties is well known. Examples of a viscosity modifier include a polymethacrylate, poly(maleic anhydride-co-styrene ester) or a polyolefin. The performance of a viscosity modifier is determined by a number of factors including (1) balanced viscosity; (2) shear stability; (3) low temperature viscometrics; and (4) molecular weight. If the molecular weight of a conventional linear viscosity modifier is increased, the viscosity increases whilst shear stability decreases and vice versa. Often this leads to formulations requiring additional amounts of lower molecular weight viscosity modifier. However, the presence of additional amounts of lower molecular weight viscosity modifier has a detrimental effect on low temperature viscometrics.
PCT application WO 00/02939 A1 discloses a process for preparing microgels and star polymers. The process includes atom transfer radical polymerisation and reversible addition fragmentation transfer polymerisation (RAFT). The microgel is formed from a prepolymer reacting with a multi-olefinic monomer and the product formed has a weight average molecular weight of 104 to 108.
U.S. Pat. No. 6,369,162 (Visger et al.) discloses a stabilised free radical polymerisation process for preparing a radial polymer containing at least 3 polymeric arms. A stabilized free radical agent is employed in the process to preserve an active polymerisation site at the terminus of the polymer. The process further includes adding at least one coupling agent to react with the polymer with an active polymerisation site at the terminus to form a radial polymer.
PCT Application 98/01478 and U.S. Pat. No. 6,642,318 disclose a free radical polymerisation process with living characteristics employing a dithio chain transfer agent. The process controls the polydispersity and molecular weight of the polymer being formed. The polymers are useful as binders in automotive and refinish coatings.
U.S. Pat. No. 6,596,899 discloses S,S′-bis-(α,α-disubstituted-α-acetic acid)-trithiocarbonate compounds as a chain transfer agent for polymerising free radical monomers. The trithiocarbonate compounds are suitable for living free radical polymerisation as well as forming telechelic polymers.
It would be advantageous to have a polymer with at least one of reduced polydispersity, modified low temperature viscometrics, modified high temperature viscometrics, modified shear stability and capable of imparting dispersant and/or viscosity modifying properties. The present invention provides a process for preparing a polymer with at least one property of reduced polydispersity, low temperature viscometrics, high temperature viscometrics, shear stability and capable of imparting dispersant and/or viscosity modifying properties; and compositions thereof.